(a) Technical Field
The present disclosure relates to a fuel cell startup apparatus and method, and more particularly, to a fuel cell startup apparatus and method which may solve problems of high-voltage generation and cathode electrode corrosion, caused by oxygen locally concentrated on a cell near a central flow distributor after long-term parking of a fuel-cell vehicle.
(b) Background Art
A main power source of a fuel-cell vehicle is a power-generating device called a ‘fuel-cell stack’. In this device, the fuel cell converts chemical energy from typically hydrogen into electric energy through an electrochemical reaction in the stack, instead of converting the chemical energy into heat through combustion.
One known type of fuel cell is a Polymer Electrolyte Membrane Fuel Cell (PEMFC), which has been widely studied as a power source for driving a vehicle. In particular, a PEMFC typically includes a Membrane Electrode Assembly (MEA) in which on both sides of an electrolyte membrane where protons move therethrough, catalyst electrode layers are attached which operates as the electrochemical reaction area. Additionally, the PEMFC also includes a Gas Diffusion Layer (GDL) which uniformly distributes reaction gas and transfers generated electric energy, and a bipolar plate which provides a path for the reaction gas for reaction and a coolant for cooling the fuel cell.
In the fuel cell, through a flow path in the bipolar plate, hydrogen as the fuel and oxygen (air) as an oxidizer are supplied to an anode and a cathode of the MEA, respectively, such that hydrogen is supplied to the anode (also called a fuel electrode, a hydrogen electrode, or an oxidation electrode) and oxygen (air) is supplied to the cathode (also called an air electrode, an oxygen electrode, or a reduction electrode).
Hydrogen supplied to the anode is resolved into protons (H+) and electrons (e−), among which only the protons selectively pass through the electrolyte membrane, which is a cation exchange membrane, and are delivered to the cathode and the electrons are delivered to the cathode through the GDL and the bipolar plate that are conductors.
In the cathode, the protons supplied through the electrolyte membrane and the electrons delivered through the bipolar plate come in contact with oxygen in the air supplied to the cathode by an air supply device, thus producing water. Due to movement of the protons in this state, the protons flow through an external conducting line, and such a flow of the electrons produce current.
As illustrated in FIG. 1, a fuel cell system applied to the fuel-cell vehicle includes a hydrogen supply device 10 for supplying hydrogen, which is operating as the fuel, to a fuel-cell stack 1, an air supply device 20 for supplying air including oxygen, which is operating as an oxidizer for electrochemical reaction, to the fuel-cell stack 1, and a hydrogen recirculation device 16 for recirculating non-reaction hydrogen exhausted from an anode outlet of the fuel-cell stack 1 into an anode inlet to reusing the hydrogen. With hydrogen recirculation, the distribution of a reagent in the stack 1 becomes uniform due to an increase in the flow rate of hydrogen in the stack 1, thus obtaining uniform cell voltage distribution and stably operating the stack 1.
Further describing the system shown in FIG. 1, high-pressure hydrogen supplied to the hydrogen supply device 10 from a hydrogen storage unit (for example, a hydrogen tank) 11 sequentially passes through a hydrogen supply valve 13 and a regulator 14 of a hydrogen supply line 12, and then is supplied to the fuel-cell stack 1 through a central flow distributor (not shown). The air supplied by an air blower 22 in the air supply device 20 passes through a humidifying device 23 and is supplied to the fuel-cell stack 1. Additionally, in at least one of an air supply line 21 and a cathode exhaust line 24 connected to cathode inlet/outlet of the fuel-cell stack 1, an air cutoff valve (not shown) is installed.
In some instances, the air cutoff value may become frozen during winter months, and thus in the fuel cell startup process, the state of the air cutoff valve is checked to determine whether the air cutoff valve is frozen.
As such, to start up the fuel cell in the fuel-cell system, hydrogen (fuel gas), which is reaction gas, air (oxidation gas) should be supplied to the fuel-cell stack. FIG. 2 is a flowchart showing a conventional fuel-cell startup process. Referring to FIG. 2, in the general fuel-cell startup process, the state of the air cutoff valve is checked in a startup standby state, and then hydrogen, which is fuel, is supplied from the hydrogen supply device to the fuel-cell stack. Thereafter, the hydrogen recirculation device (including a recirculation blower) is driven, the air supply device supplies humidified air, which is humidified by a humidifying device, to the fuel-cell stack, and then a stack voltage is checked, thus completing the startup process.
The central flow distributor connects supply and exhaust lines for hydrogen and air and supply and exhaust lines for a coolant to a manifold of the fuel-cell stack to efficiently supply and exhaust hydrogen, air, and the coolant to each unit cell of the fuel-cell stack. However, during long-term parking of the fuel-cell vehicle, external air may be introduced into the cathode of the fuel-cell stack through a crack such as a pipe connected with the fuel-cell stack, and the air (oxygen) introduced into the cathode may be introduced into the anode through the electrolyte membrane. Therefore, when the fuel cell is started up when air (oxygen) exists in the anode, a high voltage higher than a general cell voltage (for example, 1V) is formed in an interface between oxygen and hydrogen, degrading performance of the electrode.
In particular, in the case of long-term parking, air is typically introduced through the central flow distributor connected to the manifold of the fuel-cell stack, such that in a cell near the central flow distributor, the density of oxygen in the anode is higher than in other cells, and as a result, high-voltage generation is concentrated in a re-startup process, causing extensive damage to the cell near the central flow distributor.
That is, in case of long-term parking, the density of oxygen is locally quite high in the cell near the central flow distributor that is close to the pipe, intensifying high-voltage generation in that cell and thus significantly degrading performance of that cell.
Moreover, if too much high voltage is generated at startup, the thickness of the cathode electrode is reduced. In terms of one cell, the external air is mainly introduced through Air out shown in FIG. 3 and thus the density of oxygen is highest near Air out. Eventually, during re-startup, due to a high potential formed in the cathode near Air out, a carbon support can become corroded, thus reducing the thickness of the cathode electrode.
In summation, in the case of long-term parking, air is introduced into the cathode of the fuel-cell stack, and the air in the cathode moves to the anode through the electrolyte membrane, such that if the fuel cell is started up when air remains in the anode, the carbon support can become corroded, thereby reducing the thickness of the cathode electrode.
The fuel-cell stack has a structure in which unit cells are electrically connected in series, and as a result, performance of the fuel-cell stack is limited by each particular cell whose performance is degraded. Consequently, repair of the stack may be required according to performance variation between cells.